1. Field of the Invention
The present invention relates to a tracking control apparatus that makes a conversion means for recording and reproducing information follow a target track formed on a recording medium.
2. Description of the Related Art
As an example of prior art equipment for recording and reproducing information, there is an optical type recording and reproduction equipment.
Optical type recording and reproduction equipment writes and reads information by condensing a light beam and irradiating an information medium, which is called a disk hereafter, having a layer of material which can optically record and reproduce information. On the disk, there are tracks formed by grooving its substrate for recording and reproducing information. The layer of recording material is formed on the surface of the substrate by a vapor deposition. A semiconductor laser diode provides a light beam, which can be condensed to approximately the same size of its wavelength. The recording of a signal is made by modulating the amount of the light beam according to the signal and changing a physical property of the layer of recording material. The reproduction of a signal is made based on changes of light reflections from the disk, when the disk is irradiated by a constant, comparatively small power light beam.
In optical type recording and reproduction equipment, focusing control and tracking control are performed. The focusing control keeps the light beam focusing on the layer of recording material in a predetermined focusing condition. The tracking control keeps the light beam located on a target track. The following is a description of a signal, which is hereafter called a tracking error signal and expresses a locational relationship between the target track and the light beam. As shown in FIG. 2 (a), concentric pre-grooves are cut on the disk with an uneven structure having a depth of about .lambda./8, where .lambda. is the wavelength of the light beam, and are located at regular intervals, and a zone between two neighboring pre-grooves is a track where information is recorded. A disk having such a structure is called an on-land disk. FIG. 2 (b) shows how the value of the tracking error signal changes, if a locational relationship between a target track and the light beam changes as shown in FIG. 2 (a). Since the fact that the tracking error signal is obtained as in FIGS. 2 (a)-(b) for any on-land disk by means of a push-pull method which is well-known to engineers working on related fields, a description of its theory has been omitted. In FIG. 2 (a), the upper section is an exterior part of the disk, and the lower section is interior part of the disk. If the light beam shifts in the exterior direction from a track, then the tracking error signal becomes positive, and if the light beam shifts in the interior direction from a track, then the tracking error signal becomes negative. If the light beam is located in the track or if the light beam is located between two tracks, ie. on a pre-groove, then the amplitude of the tracking error signal is zero. Also, if the light beam is located on a neighborhood of a track, then the amplitude of the tracking error is proportional to the displacement of the light beam from the track. If the light beam crosses a plurality of tracks as shown in FIG. 2 (a), then the tracking error signal is shaped like a sinewave curve as shown in FIG. 2 (b).
As in other electronic equipment, the need for compact optical recording and reproduction equipment has increased recently. Therefore, compact equipment that can be put on a desk of an office has been developed. Since such equipment is usually put on a desk, it often receives a tremendous shock from the outside. On the other hand, the maximum driving force of an actuator used for tracking control is limited and usually about between 10 G to 20 G. Therefore, if a shock having a larger acceleration than the maximum driving force is given from the outside, then the tracking control apparatus can not keep the light beam following a track, and the light beam shifts out of a predetermined track, ie. a so-called track jump occurs. A response of the tracking control in the case of a track jump is shown in FIG. 3.
In FIG. 3, (a) shows the acceleration of an external shock, (c) shows a tracking error signal, and (d) shows a driving signal that drives the actuator, where the vertical axes show time. For simplicity, the external shock is assumed to be a sine curve of one period in FIG. 3. If an external shock occurs, the tracking control apparatus tries to respond and keep the light beam following a target track. However, since the external shock has a larger magnitude than the maximal driving force of the actuator, the amplitude of the tracking error signal increases, ie. a locational error of the light beam against the track increases. And finally, at time T0, track jump occurs. In FIG. 3, (b) and (e) show the time integrals of the external shock (a) and the driving signal (d), ie. the velocity of the light beam, respectively. At time T0 when track jump occurs, the light beam has the velocity V.sub.b to follow the external shock. In general, the responsibility of tracking control is limited and is at most 2 to 3 kHz. Therefore, if V.sub.b is larger than a predetermined value, then the light beam can not immediately pull in a track and move across tracks. Since, as described above, the tracking error signal has a sine curve shape at this time, the driving signal also has a sine curve shape. Therefore, even if the external shock ends at time T1, and its velocity returns to zero, the velocity of the actuator, ie. the velocity of the light beam, does not return to zero and retains values around V.sub.b as shown in FIG. 3 (e), though it repeats a fluctuation following the driving signal. In this state, it is impossible for the light beam to pull in a track, and a so-called track-skating phenomenon occurs, where the light beam continues to run across tracks. If a track-skating occurs, the light beam keeps moving until the actuator crashes against a stopper installed at the end of its movable extent, and, in the worst case, the failure of focusing control and mechanical damage, such as damage to the actuator, results.